In order to provide enhanced communication services, Third Generation Partnership Project (3GPP) cellular communication systems are designed to support a variety of different and enhanced services. One such enhanced service, named Multimedia Broadcast and Multicast Services (MBMS), is specified in 3GPP Technical Specification (3GPP TS) 36.331, Radio Resource Control, v. 12.3.0. Examples of MBMS services (evolved MBMS (eMBMS) services for LTE networks) and applications include multimedia broadcast, e.g. mobile television, audio, streamed video, etc. Some multimedia services require a high bandwidth due to the nature of the data content that is to be communicated, such as video streaming. Some multimedia services may only require a low bandwidth due to the nature of the data content that is to be communicated, such as news services. Typically, tens of channels carrying say, news, movies, sports, etc. may be broadcast simultaneously over an MBMS communication network.
For MBMS operation, a single radio transmission mode may be used, termed Multicast Broadcast Single Frequency Network (MBSFN). In this Point-To-Multipoint (PTM) transmission mode of operation, multiple communication cells synchronously transmit the same MBMS content in their respective service areas. The area covered by the participating communication cells of such a synchronised transmission is referred to as the ‘MSFSN area’. Synchronous transmission of the same MBMS content is achieved by a central network entity, termed the Multi-cell/multicast Coordination Entity (MCE), which is configured to decide both the radio resources that are used for the MBSFN transmission as well as the details of the radio resource configuration, i.e. the layer-1/layer-2 (L1/L2) configuration parameters to be used. A communication cell may participate in MBMS transmissions corresponding to different MBSFN areas, in which case MBSFN areas overlap. The radio transmission area of MBMS user data is the same as that used for the corresponding control information, i.e. the cells that participate in the transfer of the user data also participate in the transfer of the corresponding control information.
As specified in the 3GPP TS 36.331 standard, when an MBMS service (e.g. an eMBMS service) is enabled, some of the available subframes within a finite number of consecutive radio frames (or system frames) are allocated to the MBMS service. This MBMS subframe allocation can be repeated periodically. The periodicity can be set to one, two, four, eight, sixteen or thirty-two radio frames. The standard further specifies ways to extend the periodicity to 64, 128 and 256 radio frames. These defined procedures give flexibility to the operator to adjust the bandwidth allocation for MBMS services.
Normally, before the establishment of an MBMS service, a subframe allocation for the MBMS service is semi-statically configured ie. MBMS services are scheduled in the order in which they are listed on the multicast control channel (MCCH). Once configured, the MBMS subframe pattern allocated typically does not change and is indicated in a System Information Block 2 (SIB2) broadcast message as part of the Multicast Broadcast Single-Frequency Network (MBSFN) SubframeConfiguration Information Element (IE). Depending on the needs, an MBMS service can be configured on any number and/or any combination of subframes (with certain exceptions) within a radio frame, up to a maximum percentage (typically 60%) of the available bandwidth. These MBMS subframes (or MBSFN subframes) are only available for MBMS transmissions once the radio resources are allocated for broadcast/multi-cast services, i.e., they typically can't be used for unicast traffic like for example Voice over Long Term Evolution (VoLTE) or File Transfer Protocol (FTP) services.
A drawback of multicast/broadcast communications transmissions (such as MBMS) is that once configured, the (periodic) resources allocated do not change. This may result in conflicts with other resources already configured and/or needed for other services or activities at the UE. In the context of UE mobility in LTE networks for example, the E-UTRAN is required to allocate time and/or radio resources for a UE to perform radio frequency measurements on various channels (e.g. on the frequency channel used by its serving cell, other E-UTRA frequencies and/or frequencies used by other Radio Access Technologies). Since UEs typically employ a single transceiver, it is not possible for the UE to perform these inter-frequency and inter-RAT measurements, whilst it is engaged in a (unicast or multicast) data transmission with the serving cell, as the transceiver is fully occupied in its communications on a particular frequency channel. To overcome this, the E-UTRAN in 3GPP has been designed to configure time segments in which it does not schedule any downlink data transfer or uplink grants. During these periodically appearing segments, also referred to as “measurement gaps (or “mobility gaps” in the context of handovers), the UE is able to perform the required (periodic) measurements on the serving or other frequency channels, as detailed above. Depending on the bandwidth required, when an MBMS service is initiated, the periodic subframe resource configuration allocated for the MBMS service may conflict with measurement gaps a UE needs to perform radio frequency measurements.
Another potential area of conflict is in relation to Discontinuous Reception (DRX) resources. As is well known, DRX reduces battery consumption in UE by limiting the time when Downlink (DL) receptions need to be monitored by the UE. In other words, the UE can only be scheduled when the UE monitors the PDCCH. This implies that the UE can only be scheduled resources during (periodic) segments of time when the UE is awake (also known as the “active time”, “wake time” or “on duration”). In LTE networks, the DRX resource configuration is controlled by the eNB and is specific to each UE. Once a DRX resource configuration has been received from the eNB, UEs normally use a timer referred to as the “onDuration” timer to determine when to be awake. To spread the scheduling load in time, the eNB will typically configure UEs with a particular (and preferably different) DRX configuration such that UEs do not have simultaneous wake times. When an MBMS service is established, it is quite possible that the subframe resource configuration allocated for the MBMS service conflicts with the UE's DRX resource configuration.